Leadless cardiac pacemakers (LCPs) represent a revolutionary leap forward in cardiac pacing technology via its circumvention of transvenous leads. Current LCP lithium/CFX batteries are ~0.6 cc resulting in an overall 1 cc LCP device. This Phase I effort will demonstrate the feasibility of a 0.1 cc betavoltaic battery for LCPs with a 20-year lifetime (~2x lifetime of current LCP batteries) enabling LCPs with a volumetric size of ?0.5 cc. This size reduction and increased longevity will allow for 2-3 implants over a patient?s lifetime with minimal invasive overhead. Furthermore, LCPs are currently limited to single chamber pacing, representing only 10-20% of the current transvenous implant market. Dual chamber and multi-chamber leadless pacing also require a size reduction of the LCP to meet the smaller volume space associated with the atria. A betavoltaic battery with a 0.1 cc form factor and a reliable 20-year life will facilitate mainstream use of LCPs while challenging traditional pacemakers. Phase I will demonstrate feasibility via the construction of stackable 10-15 micron thick III-V betavoltaic cells that utilize a new high beta-flux, tritium metal hydride film. Preliminary data shows that ? 200 microwatts per cc (target power density for a ~ 0.1 cc LCP battery) may be reached if the betavoltaic cell (i.e. tritium film coupled to the semiconductor cell) is thinned down to stackable 10-micron layers. The work in Phase I will lay the foundation for the design of the stackable cell unit to be developed in Phase II, which will result in a battery prototype for testing and integration by a pacemaker corporate partner. The design will account for electrical parallel stacking of n/p and p/n cells, I-V characteristic behavior from Phase 1 cells, packaging considerations, LCP manufacturer input, and guidelines from regulatory agencies. Tritium betavoltaic technology is a solid-state power source that does not lose its energy density with volumetric reduction as in the case of lithium batteries. Its principles of operation are similar to a solar cell, but in lieu of photons impinging on the semiconductor cell, the electrons from the radioisotope?s beta decay are utilized. Specific Aim 1: Develop n/p and p/n wide bandgap diode junctions that are thin and stackable. Task 1: Metal- Organic Chemical Vapour Deposition (MOCVD) growth of n/p and p/n junctions. Task 2: Thin down substrate and deposit back metal. Milestones: Cells with ?15 microns of thickness. Dark I-V measurements yielding ?50 nanoamps/cm2 at 1.1 Volts in Tasks 1 and 1.0 Volts in Task 2. Specific Aim 2: Deposit metal hydride on cell surface to demonstrate thin and stackable betavoltaic cells. Task 3: Deposit metal hydride junction active area and investigate dark I-V properties and loading capacity. The metal hydride will be loaded with a tritium surrogate of protium or deuterium. Milestones: Dark I-V criteria of ?50 nanoamps/cm2 at 0.9 Volts and a loading capacity of 6% or higher by weight of hydrogen to metal. These two specific aims will demonstrate the feasibility of the betavoltaic layers to be stacked in Phase II; resulting in a betavoltaic battery for testing and integration into a revolutionary ultra-small LCP with approximately double the lifetime of current batteries.